1. Field of the Invention
The present invention provides an optical system, and more particularly, an optical system capable of generating a non-radially symmetrical light beam.
2. Descriptions of the Related Art
A digital light processing (DLP) projector generally comprises an optical system, a digital micromirror device (DMD) and a projection lens assembly. The optical system is adapted to generate an incident light beam to be projected to the DMD. The DMD comprises a plurality of micromirrors. The incident light beam projected to the micromirrors is reflected by the micromirrors. By controlling the deflecting angle of each of the micromirrors, specific parts of the incident light beam can be selected to project and reflect onto the projection lens assembly.
Furthermore, in reference to FIG. 1, each micromirror 7 of the DMD has two statuses: an On status and an Off status, which are distinguished by the tilt angle of the micromirror 7. The micromirror 7 has a tilt angle of about 12° in the On status and a tilt angle of about −12° in the Off status. An incident light beam 81 generated by the optical system is reflected by the micromirror 7 into a first reflected light beam 71 and a second reflected light beam 72 with different directions in different statuses respectively.
Furthermore, the incident light beam 81 also illuminates the planar structure of the DMD (e.g., the plane between two adjacent micromirrors 7, not shown in the figure) and is then reflected by the planar structure into another reflected light beam which is called a stray light 73.
Ideally, only the first reflected light beam 71 corresponding to the On status can propagate through the aperture 91 of the projection lens assembly 9 into the projection lens assembly 9 and is projected outwards by the projection lens assembly 9, while the second reflected light beam 72 and the stray light 73 cannot propagate through the aperture 91. However, in practice, a part of the stray light 73 may also propagate into the projection lens assembly 9 for the following reasons: the stray light 73 has too large of a divergence angle α in the direction 75 perpendicular to the rotation axis 74 of the micromirror 7, so the stray light 73 can propagate into the aperture 91 in the same direction 75. The situation would decrease the contrast of images projected by the projection lens assembly 9.
In order to overcome this shortcoming, some solutions have been proposed, for example, in U.S. Pat. No. 7,246,923 and U.S. Pat. No. 7,101,050. In these solutions, a non-radially symmetrical light beam can be generated by the optical system to illuminate the DMD so that the reflected light beams reflected by the micromirrors and the stray light reflected by the planar structure are all non-radially symmetrical; and in this case, the aperture of the projection lens assembly is also non-radially symmetrical.
The non-radially symmetrical reflected light beams and stray light all have a small divergence angle in a direction (e.g., the direction 75 shown in FIG. 1) perpendicular to the rotation axis of the micromirror. As a result, the stray light is less likely to propagate into the aperture of the projection lens assembly and, accordingly, the contrast of images projected by the projection lens assembly can be improved.
On the other hand, in the direction parallel to the rotation axis of the micromirror, the non-radially symmetrical reflected light beams have a relatively large divergence angle, so the reflected light beams have a relatively large Etendue. Consequently, in the On status, the reflected light beams with a relatively large Etendue can propagate into the aperture to result in an improved brightness of the images projected by the projection lens assembly.
In other words, the non-radially symmetrical nature of the light beams generated by the optical system is beneficial to both the contrast and the brightness of the images projected by the projection lens assembly.
However, in the aforesaid solutions, some special optical components such as a collector or an integrator are used in the optical system, which may increase the manufacturing cost of the optical system.
In view of this, an urgent need exists in the art to provide an optical system that can overcome the shortcoming described above.